[2/3] x86/split_lock: Refactor and export handle_user_split_lock() for KVM

Commit Message

Sean Christopherson April 2, 2020, 3:55 p.m. UTC

From: Xiaoyao Li <[e-mail]>

In the future, KVM will use handle_user_split_lock() to handle #AC
caused by split lock in guest. Due to the fact that KVM doesn't have
a @regs context and will pre-check EFLAGS.AC, move the EFLAGS.AC check
to do_alignment_check().

Suggested-by: Sean Christopherson <[e-mail]>
Signed-off-by: Xiaoyao Li <[e-mail]>
Reviewed-by: Tony Luck <[e-mail]>
Signed-off-by: Sean Christopherson <[e-mail]>
---
 arch/x86/include/asm/cpu.h  | 4 ++--
 arch/x86/kernel/cpu/intel.c | 7 ++++---
 arch/x86/kernel/traps.c     | 2 +-
 3 files changed, 7 insertions(+), 6 deletions(-)

Comments

Thomas Gleixner April 2, 2020, 5:01 p.m. UTC | #1

Sean Christopherson <[e-mail]> writes:
> From: Xiaoyao Li <[e-mail]>
>
> In the future, KVM will use handle_user_split_lock() to handle #AC
> caused by split lock in guest. Due to the fact that KVM doesn't have
> a @regs context and will pre-check EFLAGS.AC, move the EFLAGS.AC check
> to do_alignment_check().
>
> Suggested-by: Sean Christopherson <[e-mail]>
> Signed-off-by: Xiaoyao Li <[e-mail]>
> Reviewed-by: Tony Luck <[e-mail]>
> Signed-off-by: Sean Christopherson <[e-mail]>
> ---
>  arch/x86/include/asm/cpu.h  | 4 ++--
>  arch/x86/kernel/cpu/intel.c | 7 ++++---
>  arch/x86/kernel/traps.c     | 2 +-
>  3 files changed, 7 insertions(+), 6 deletions(-)
>
> diff --git a/arch/x86/include/asm/cpu.h b/arch/x86/include/asm/cpu.h
> index ff6f3ca649b3..ff567afa6ee1 100644
> --- a/arch/x86/include/asm/cpu.h
> +++ b/arch/x86/include/asm/cpu.h
> @@ -43,11 +43,11 @@ unsigned int x86_stepping(unsigned int sig);
>  #ifdef CONFIG_CPU_SUP_INTEL
>  extern void __init cpu_set_core_cap_bits(struct cpuinfo_x86 *c);
>  extern void switch_to_sld(unsigned long tifn);
> -extern bool handle_user_split_lock(struct pt_regs *regs, long error_code);
> +extern bool handle_user_split_lock(unsigned long ip);
>  #else
>  static inline void __init cpu_set_core_cap_bits(struct cpuinfo_x86 *c) {}
>  static inline void switch_to_sld(unsigned long tifn) {}
> -static inline bool handle_user_split_lock(struct pt_regs *regs, long error_code)
> +static inline bool handle_user_split_lock(unsigned long ip)

This is necessary because VMX can be compiled without CPU_SUP_INTEL?

>  {
>  	return false;
>  }
> diff --git a/arch/x86/kernel/cpu/intel.c b/arch/x86/kernel/cpu/intel.c
> index 9a26e972cdea..7688f51aabdb 100644
> --- a/arch/x86/kernel/cpu/intel.c
> +++ b/arch/x86/kernel/cpu/intel.c
> @@ -1066,13 +1066,13 @@ static void split_lock_init(void)
>  	split_lock_verify_msr(sld_state != sld_off);
>  }
>  
> -bool handle_user_split_lock(struct pt_regs *regs, long error_code)
> +bool handle_user_split_lock(unsigned long ip)
>  {
> -	if ((regs->flags & X86_EFLAGS_AC) || sld_state == sld_fatal)
> +	if (sld_state == sld_fatal)
>  		return false;
>  
>  	pr_warn_ratelimited("#AC: %s/%d took a split_lock trap at address: 0x%lx\n",
> -			    current->comm, current->pid, regs->ip);
> +			    current->comm, current->pid, ip);

So this returns true even in the case that sld_state == off.

Should never happen, but I rather have an extra check and be both
verbose and correct. See the variant I did.

Thanks,

        tglx

Sean Christopherson April 2, 2020, 5:19 p.m. UTC | #2

On Thu, Apr 02, 2020 at 07:01:56PM +0200, Thomas Gleixner wrote:
> Sean Christopherson <[e-mail]> writes:
> > From: Xiaoyao Li <[e-mail]>
> >
> > In the future, KVM will use handle_user_split_lock() to handle #AC
> > caused by split lock in guest. Due to the fact that KVM doesn't have
> > a @regs context and will pre-check EFLAGS.AC, move the EFLAGS.AC check
> > to do_alignment_check().
> >
> > Suggested-by: Sean Christopherson <[e-mail]>
> > Signed-off-by: Xiaoyao Li <[e-mail]>
> > Reviewed-by: Tony Luck <[e-mail]>
> > Signed-off-by: Sean Christopherson <[e-mail]>
> > ---
> >  arch/x86/include/asm/cpu.h  | 4 ++--
> >  arch/x86/kernel/cpu/intel.c | 7 ++++---
> >  arch/x86/kernel/traps.c     | 2 +-
> >  3 files changed, 7 insertions(+), 6 deletions(-)
> >
> > diff --git a/arch/x86/include/asm/cpu.h b/arch/x86/include/asm/cpu.h
> > index ff6f3ca649b3..ff567afa6ee1 100644
> > --- a/arch/x86/include/asm/cpu.h
> > +++ b/arch/x86/include/asm/cpu.h
> > @@ -43,11 +43,11 @@ unsigned int x86_stepping(unsigned int sig);
> >  #ifdef CONFIG_CPU_SUP_INTEL
> >  extern void __init cpu_set_core_cap_bits(struct cpuinfo_x86 *c);
> >  extern void switch_to_sld(unsigned long tifn);
> > -extern bool handle_user_split_lock(struct pt_regs *regs, long error_code);
> > +extern bool handle_user_split_lock(unsigned long ip);
> >  #else
> >  static inline void __init cpu_set_core_cap_bits(struct cpuinfo_x86 *c) {}
> >  static inline void switch_to_sld(unsigned long tifn) {}
> > -static inline bool handle_user_split_lock(struct pt_regs *regs, long error_code)
> > +static inline bool handle_user_split_lock(unsigned long ip)
> 
> This is necessary because VMX can be compiled without CPU_SUP_INTEL?

Ya, it came about when cleaning up the IA32_FEATURE_CONTROL MSR handling
to consolidate duplicate code.

config KVM_INTEL
        tristate "KVM for Intel (and compatible) processors support"
        depends on KVM && IA32_FEAT_CTL

config IA32_FEAT_CTL
        def_bool y
        depends on CPU_SUP_INTEL || CPU_SUP_CENTAUR || CPU_SUP_ZHAOXIN

Thomas Gleixner April 2, 2020, 7:06 p.m. UTC | #3

Sean Christopherson <[e-mail]> writes:
> On Thu, Apr 02, 2020 at 07:01:56PM +0200, Thomas Gleixner wrote:
>> >  static inline void __init cpu_set_core_cap_bits(struct cpuinfo_x86 *c) {}
>> >  static inline void switch_to_sld(unsigned long tifn) {}
>> > -static inline bool handle_user_split_lock(struct pt_regs *regs, long error_code)
>> > +static inline bool handle_user_split_lock(unsigned long ip)
>> 
>> This is necessary because VMX can be compiled without CPU_SUP_INTEL?
>
> Ya, it came about when cleaning up the IA32_FEATURE_CONTROL MSR handling
> to consolidate duplicate code.
>
> config KVM_INTEL
>         tristate "KVM for Intel (and compatible) processors support"
>         depends on KVM && IA32_FEAT_CTL
>
> config IA32_FEAT_CTL
>         def_bool y
>         depends on CPU_SUP_INTEL || CPU_SUP_CENTAUR || CPU_SUP_ZHAOXIN

Ah, indeed. So something like the below would make sense. Hmm?

Of course that can be mangled into Xiaoyao's patches, I'm not worried
about my patch count :)

Aside of that I really wish Intel HW folks had indicated the source of
the #AC via the error code. It can only be 0 or 1 for the regular #AC so
there would have been 31 bits to chose from.

Thanks,

        tglx

8<----------------
--- a/arch/x86/include/asm/cpu.h
+++ b/arch/x86/include/asm/cpu.h
@@ -43,14 +43,14 @@ unsigned int x86_stepping(unsigned int s
 #ifdef CONFIG_CPU_SUP_INTEL
 extern void __init cpu_set_core_cap_bits(struct cpuinfo_x86 *c);
 extern void switch_to_sld(unsigned long tifn);
-extern bool handle_user_split_lock(struct pt_regs *regs, long error_code);
+extern int handle_ac_split_lock(unsigned long ip);
 extern void split_lock_validate_module_text(struct module *me, void *text, void *text_end);
 #else
 static inline void __init cpu_set_core_cap_bits(struct cpuinfo_x86 *c) {}
 static inline void switch_to_sld(unsigned long tifn) {}
-static inline bool handle_user_split_lock(struct pt_regs *regs, long error_code)
+static int handle_ac_split_lock(unsigned long ip)
 {
-	return false;
+	return -ENOSYS;
 }
 static inline void split_lock_validate_module_text(struct module *me, void *text, void *text_end) {}
 #endif

--- a/arch/x86/kernel/cpu/intel.c
+++ b/arch/x86/kernel/cpu/intel.c
@@ -1102,13 +1102,20 @@ static void split_lock_init(void)
 	split_lock_verify_msr(sld_state != sld_off);
 }
 
-bool handle_user_split_lock(struct pt_regs *regs, long error_code)
+int handle_ac_split_lock(unsigned long ip)
 {
-	if ((regs->flags & X86_EFLAGS_AC) || sld_state == sld_fatal)
-		return false;
+	switch (sld_state) {
+	case sld_warn:
+		break;
+	case sld_off:
+		pr_warn_once("#AC: Spurious trap at address: 0x%lx\n", ip);
+		return -ENOSYS;
+	case sld_fatal:
+		return -EFAULT;
+	}
 
 	pr_warn_ratelimited("#AC: %s/%d took a split_lock trap at address: 0x%lx\n",
-			    current->comm, current->pid, regs->ip);
+			    current->comm, current->pid, ip);
 
 	/*
 	 * Disable the split lock detection for this task so it can make
@@ -1117,8 +1124,9 @@ bool handle_user_split_lock(struct pt_re
 	 */
 	sld_update_msr(false);
 	set_tsk_thread_flag(current, TIF_SLD);
-	return true;
+	return 0;
 }
+EXPORT_SYMBOL_GPL(handle_ac_split_lock);
 
 /*
  * This function is called only when switching between tasks with
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -304,7 +304,7 @@ dotraplinkage void do_alignment_check(st
 
 	local_irq_enable();
 
-	if (handle_user_split_lock(regs, error_code))
+	if (!(regs->flags & X86_EFLAGS_AC) && !handle_ac_split_lock(regs->ip))
 		return;
 
 	do_trap(X86_TRAP_AC, SIGBUS, "alignment check", regs,
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -65,6 +65,7 @@
 
 MODULE_AUTHOR("Qumranet");
 MODULE_LICENSE("GPL");
+MODULE_INFO(sld_safe, "Y");
 
 #ifdef MODULE
 static const struct x86_cpu_id vmx_cpu_id[] = {
@@ -4623,6 +4624,22 @@ static int handle_machine_check(struct k
 	return 1;
 }
 
+static bool guest_handles_ac(struct kvm_vcpu *vcpu)
+{
+	/*
+	 * If guest has alignment checking enabled in CR0 and activated in
+	 * eflags, then the #AC originated from CPL3 and the guest is able
+	 * to handle it. It does not matter whether this is a regular or
+	 * a split lock operation induced #AC.
+	 */
+	if (vmx_get_cpl(vcpu) == 3 && kvm_read_cr0_bits(vcpu, X86_CR0_AM) &&
+	    kvm_get_rflags(vcpu) & X86_EFLAGS_AC)
+		return true;
+
+	/* Add guest SLD handling checks here once it's supported */
+	return false;
+}
+
 static int handle_exception_nmi(struct kvm_vcpu *vcpu)
 {
 	struct vcpu_vmx *vmx = to_vmx(vcpu);
@@ -4630,6 +4647,7 @@ static int handle_exception_nmi(struct k
 	u32 intr_info, ex_no, error_code;
 	unsigned long cr2, rip, dr6;
 	u32 vect_info;
+	int err;
 
 	vect_info = vmx->idt_vectoring_info;
 	intr_info = vmx->exit_intr_info;
@@ -4688,9 +4706,6 @@ static int handle_exception_nmi(struct k
 		return handle_rmode_exception(vcpu, ex_no, error_code);
 
 	switch (ex_no) {
-	case AC_VECTOR:
-		kvm_queue_exception_e(vcpu, AC_VECTOR, error_code);
-		return 1;
 	case DB_VECTOR:
 		dr6 = vmcs_readl(EXIT_QUALIFICATION);
 		if (!(vcpu->guest_debug &
@@ -4719,6 +4734,29 @@ static int handle_exception_nmi(struct k
 		kvm_run->debug.arch.pc = vmcs_readl(GUEST_CS_BASE) + rip;
 		kvm_run->debug.arch.exception = ex_no;
 		break;
+	case AC_VECTOR:
+		if (guest_handles_ac(vcpu)) {
+			kvm_queue_exception_e(vcpu, AC_VECTOR, error_code);
+			return 1;
+		}
+		/*
+		 * Handle #AC caused by split lock detection. If the host
+		 * mode is sld_warn, then it warns, marks current with
+		 * TIF_SLD and disables split lock detection. So the guest
+		 * can just continue.
+		 *
+		 * If the host mode is fatal, the handling code warned. Let
+		 * qemu kill itself.
+		 *
+		 * If the host mode is off, then this #AC is bonkers and
+		 * something is badly wrong. Let it fail as well.
+		 */
+		err = handle_ac_split_lock(kvm_rip_read(vcpu));
+		if (!err)
+			return 1;
+		/* Propagate the error type to user space */
+		error_code = err == -EFAULT ? 0x100 : 0x200;
+		fallthrough;
 	default:
 		kvm_run->exit_reason = KVM_EXIT_EXCEPTION;
 		kvm_run->ex.exception = ex_no;

Xiaoyao Li April 10, 2020, 4:39 a.m. UTC | #4

On 4/3/2020 3:06 AM, Thomas Gleixner wrote:
> Sean Christopherson <[e-mail]> writes:
>> On Thu, Apr 02, 2020 at 07:01:56PM +0200, Thomas Gleixner wrote:
>>>>   static inline void __init cpu_set_core_cap_bits(struct cpuinfo_x86 *c) {}
>>>>   static inline void switch_to_sld(unsigned long tifn) {}
>>>> -static inline bool handle_user_split_lock(struct pt_regs *regs, long error_code)
>>>> +static inline bool handle_user_split_lock(unsigned long ip)
>>>
>>> This is necessary because VMX can be compiled without CPU_SUP_INTEL?
>>
>> Ya, it came about when cleaning up the IA32_FEATURE_CONTROL MSR handling
>> to consolidate duplicate code.
>>
>> config KVM_INTEL
>>          tristate "KVM for Intel (and compatible) processors support"
>>          depends on KVM && IA32_FEAT_CTL
>>
>> config IA32_FEAT_CTL
>>          def_bool y
>>          depends on CPU_SUP_INTEL || CPU_SUP_CENTAUR || CPU_SUP_ZHAOXIN
> 
> Ah, indeed. So something like the below would make sense. Hmm?
> 
> Of course that can be mangled into Xiaoyao's patches, I'm not worried
> about my patch count :)
> 

I don't mind using yours in my next version.

Hi Paolo,

Are you OK with the kvm part below?

If no objection, I can spin the next version using tglx's.

> 
> 8<----------------
> --- a/arch/x86/include/asm/cpu.h
> +++ b/arch/x86/include/asm/cpu.h
> @@ -43,14 +43,14 @@ unsigned int x86_stepping(unsigned int s
>   #ifdef CONFIG_CPU_SUP_INTEL
>   extern void __init cpu_set_core_cap_bits(struct cpuinfo_x86 *c);
>   extern void switch_to_sld(unsigned long tifn);
> -extern bool handle_user_split_lock(struct pt_regs *regs, long error_code);
> +extern int handle_ac_split_lock(unsigned long ip);
>   extern void split_lock_validate_module_text(struct module *me, void *text, void *text_end);
>   #else
>   static inline void __init cpu_set_core_cap_bits(struct cpuinfo_x86 *c) {}
>   static inline void switch_to_sld(unsigned long tifn) {}
> -static inline bool handle_user_split_lock(struct pt_regs *regs, long error_code)
> +static int handle_ac_split_lock(unsigned long ip)
>   {
> -	return false;
> +	return -ENOSYS;
>   }
>   static inline void split_lock_validate_module_text(struct module *me, void *text, void *text_end) {}
>   #endif
> 
> --- a/arch/x86/kernel/cpu/intel.c
> +++ b/arch/x86/kernel/cpu/intel.c
> @@ -1102,13 +1102,20 @@ static void split_lock_init(void)
>   	split_lock_verify_msr(sld_state != sld_off);
>   }
>   
> -bool handle_user_split_lock(struct pt_regs *regs, long error_code)
> +int handle_ac_split_lock(unsigned long ip)
>   {
> -	if ((regs->flags & X86_EFLAGS_AC) || sld_state == sld_fatal)
> -		return false;
> +	switch (sld_state) {
> +	case sld_warn:
> +		break;
> +	case sld_off:
> +		pr_warn_once("#AC: Spurious trap at address: 0x%lx\n", ip);
> +		return -ENOSYS;
> +	case sld_fatal:
> +		return -EFAULT;
> +	}
>   
>   	pr_warn_ratelimited("#AC: %s/%d took a split_lock trap at address: 0x%lx\n",
> -			    current->comm, current->pid, regs->ip);
> +			    current->comm, current->pid, ip);
>   
>   	/*
>   	 * Disable the split lock detection for this task so it can make
> @@ -1117,8 +1124,9 @@ bool handle_user_split_lock(struct pt_re
>   	 */
>   	sld_update_msr(false);
>   	set_tsk_thread_flag(current, TIF_SLD);
> -	return true;
> +	return 0;
>   }
> +EXPORT_SYMBOL_GPL(handle_ac_split_lock);
>   
>   /*
>    * This function is called only when switching between tasks with
> --- a/arch/x86/kernel/traps.c
> +++ b/arch/x86/kernel/traps.c
> @@ -304,7 +304,7 @@ dotraplinkage void do_alignment_check(st
>   
>   	local_irq_enable();
>   
> -	if (handle_user_split_lock(regs, error_code))
> +	if (!(regs->flags & X86_EFLAGS_AC) && !handle_ac_split_lock(regs->ip))
>   		return;
>   
>   	do_trap(X86_TRAP_AC, SIGBUS, "alignment check", regs,
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -65,6 +65,7 @@
>   
>   MODULE_AUTHOR("Qumranet");
>   MODULE_LICENSE("GPL");
> +MODULE_INFO(sld_safe, "Y");
>   
>   #ifdef MODULE
>   static const struct x86_cpu_id vmx_cpu_id[] = {
> @@ -4623,6 +4624,22 @@ static int handle_machine_check(struct k
>   	return 1;
>   }
>   
> +static bool guest_handles_ac(struct kvm_vcpu *vcpu)
> +{
> +	/*
> +	 * If guest has alignment checking enabled in CR0 and activated in
> +	 * eflags, then the #AC originated from CPL3 and the guest is able
> +	 * to handle it. It does not matter whether this is a regular or
> +	 * a split lock operation induced #AC.
> +	 */
> +	if (vmx_get_cpl(vcpu) == 3 && kvm_read_cr0_bits(vcpu, X86_CR0_AM) &&
> +	    kvm_get_rflags(vcpu) & X86_EFLAGS_AC)
> +		return true;
> +
> +	/* Add guest SLD handling checks here once it's supported */
> +	return false;
> +}
> +
>   static int handle_exception_nmi(struct kvm_vcpu *vcpu)
>   {
>   	struct vcpu_vmx *vmx = to_vmx(vcpu);
> @@ -4630,6 +4647,7 @@ static int handle_exception_nmi(struct k
>   	u32 intr_info, ex_no, error_code;
>   	unsigned long cr2, rip, dr6;
>   	u32 vect_info;
> +	int err;
>   
>   	vect_info = vmx->idt_vectoring_info;
>   	intr_info = vmx->exit_intr_info;
> @@ -4688,9 +4706,6 @@ static int handle_exception_nmi(struct k
>   		return handle_rmode_exception(vcpu, ex_no, error_code);
>   
>   	switch (ex_no) {
> -	case AC_VECTOR:
> -		kvm_queue_exception_e(vcpu, AC_VECTOR, error_code);
> -		return 1;
>   	case DB_VECTOR:
>   		dr6 = vmcs_readl(EXIT_QUALIFICATION);
>   		if (!(vcpu->guest_debug &
> @@ -4719,6 +4734,29 @@ static int handle_exception_nmi(struct k
>   		kvm_run->debug.arch.pc = vmcs_readl(GUEST_CS_BASE) + rip;
>   		kvm_run->debug.arch.exception = ex_no;
>   		break;
> +	case AC_VECTOR:
> +		if (guest_handles_ac(vcpu)) {
> +			kvm_queue_exception_e(vcpu, AC_VECTOR, error_code);
> +			return 1;
> +		}
> +		/*
> +		 * Handle #AC caused by split lock detection. If the host
> +		 * mode is sld_warn, then it warns, marks current with
> +		 * TIF_SLD and disables split lock detection. So the guest
> +		 * can just continue.
> +		 *
> +		 * If the host mode is fatal, the handling code warned. Let
> +		 * qemu kill itself.
> +		 *
> +		 * If the host mode is off, then this #AC is bonkers and
> +		 * something is badly wrong. Let it fail as well.
> +		 */
> +		err = handle_ac_split_lock(kvm_rip_read(vcpu));
> +		if (!err)
> +			return 1;
> +		/* Propagate the error type to user space */
> +		error_code = err == -EFAULT ? 0x100 : 0x200;
> +		fallthrough;
>   	default:
>   		kvm_run->exit_reason = KVM_EXIT_EXCEPTION;
>   		kvm_run->ex.exception = ex_no;
>

Paolo Bonzini April 10, 2020, 10:21 a.m. UTC | #5

On 10/04/20 06:39, Xiaoyao Li wrote:
> 
>   +static bool guest_handles_ac(struct kvm_vcpu *vcpu)
> +{
> +    /*
> +     * If guest has alignment checking enabled in CR0 and activated in
> +     * eflags, then the #AC originated from CPL3 and the guest is able
> +     * to handle it. It does not matter whether this is a regular or
> +     * a split lock operation induced #AC.
> +     */
> +    if (vmx_get_cpl(vcpu) == 3 && kvm_read_cr0_bits(vcpu, X86_CR0_AM) &&
> +        kvm_get_rflags(vcpu) & X86_EFLAGS_AC)
> +        return true;
> +
> +    /* Add guest SLD handling checks here once it's supported */
> +    return false;
> +}
> +
>   static int handle_exception_nmi(struct kvm_vcpu *vcpu)
>   {
>       struct vcpu_vmx *vmx = to_vmx(vcpu);
> @@ -4630,6 +4647,7 @@ static int handle_exception_nmi(struct k
>       u32 intr_info, ex_no, error_code;
>       unsigned long cr2, rip, dr6;
>       u32 vect_info;
> +    int err;
>         vect_info = vmx->idt_vectoring_info;
>       intr_info = vmx->exit_intr_info;
> @@ -4688,9 +4706,6 @@ static int handle_exception_nmi(struct k
>           return handle_rmode_exception(vcpu, ex_no, error_code);
>         switch (ex_no) {
> -    case AC_VECTOR:
> -        kvm_queue_exception_e(vcpu, AC_VECTOR, error_code);
> -        return 1;
>       case DB_VECTOR:
>           dr6 = vmcs_readl(EXIT_QUALIFICATION);
>           if (!(vcpu->guest_debug &
> @@ -4719,6 +4734,29 @@ static int handle_exception_nmi(struct k
>           kvm_run->debug.arch.pc = vmcs_readl(GUEST_CS_BASE) + rip;
>           kvm_run->debug.arch.exception = ex_no;
>           break;
> +    case AC_VECTOR:
> +        if (guest_handles_ac(vcpu)) {
> +            kvm_queue_exception_e(vcpu, AC_VECTOR, error_code);
> +            return 1;
> +        }
> +        /*
> +         * Handle #AC caused by split lock detection. If the host
> +         * mode is sld_warn, then it warns, marks current with
> +         * TIF_SLD and disables split lock detection. So the guest
> +         * can just continue.
> +         *
> +         * If the host mode is fatal, the handling code warned. Let
> +         * qemu kill itself.
> +         *
> +         * If the host mode is off, then this #AC is bonkers and
> +         * something is badly wrong. Let it fail as well.
> +         */
> +        err = handle_ac_split_lock(kvm_rip_read(vcpu));
> +        if (!err)
> +            return 1;
> +        /* Propagate the error type to user space */
> +        error_code = err == -EFAULT ? 0x100 : 0x200;
> +        fallthrough;


Acked-by: Paolo Bonzini <[e-mail]>

Patch

diff --git a/arch/x86/include/asm/cpu.h b/arch/x86/include/asm/cpu.h
index ff6f3ca649b3..ff567afa6ee1 100644
--- a/arch/x86/include/asm/cpu.h
+++ b/arch/x86/include/asm/cpu.h
@@ -43,11 +43,11 @@  unsigned int x86_stepping(unsigned int sig);
 #ifdef CONFIG_CPU_SUP_INTEL
 extern void __init cpu_set_core_cap_bits(struct cpuinfo_x86 *c);
 extern void switch_to_sld(unsigned long tifn);
-extern bool handle_user_split_lock(struct pt_regs *regs, long error_code);
+extern bool handle_user_split_lock(unsigned long ip);
 #else
 static inline void __init cpu_set_core_cap_bits(struct cpuinfo_x86 *c) {}
 static inline void switch_to_sld(unsigned long tifn) {}
-static inline bool handle_user_split_lock(struct pt_regs *regs, long error_code)
+static inline bool handle_user_split_lock(unsigned long ip)
 {
 	return false;
 }
diff --git a/arch/x86/kernel/cpu/intel.c b/arch/x86/kernel/cpu/intel.c
index 9a26e972cdea..7688f51aabdb 100644
--- a/arch/x86/kernel/cpu/intel.c
+++ b/arch/x86/kernel/cpu/intel.c
@@ -1066,13 +1066,13 @@  static void split_lock_init(void)
 	split_lock_verify_msr(sld_state != sld_off);
 }
 
-bool handle_user_split_lock(struct pt_regs *regs, long error_code)
+bool handle_user_split_lock(unsigned long ip)
 {
-	if ((regs->flags & X86_EFLAGS_AC) || sld_state == sld_fatal)
+	if (sld_state == sld_fatal)
 		return false;
 
 	pr_warn_ratelimited("#AC: %s/%d took a split_lock trap at address: 0x%lx\n",
-			    current->comm, current->pid, regs->ip);
+			    current->comm, current->pid, ip);
 
 	/*
 	 * Disable the split lock detection for this task so it can make
@@ -1083,6 +1083,7 @@  bool handle_user_split_lock(struct pt_regs *regs, long error_code)
 	set_tsk_thread_flag(current, TIF_SLD);
 	return true;
 }
+EXPORT_SYMBOL_GPL(handle_user_split_lock);
 
 /*
  * This function is called only when switching between tasks with
diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index d54cffdc7cac..55902c5bf0aa 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -304,7 +304,7 @@  dotraplinkage void do_alignment_check(struct pt_regs *regs, long error_code)
 
 	local_irq_enable();
 
-	if (handle_user_split_lock(regs, error_code))
+	if (!(regs->flags & X86_EFLAGS_AC) && handle_user_split_lock(regs->ip))
 		return;
 
 	do_trap(X86_TRAP_AC, SIGBUS, "alignment check", regs,